In order to achieve high-speed downlink transmission in a Wideband Code Division Multiple Access (W-CDMA) system compliant to the 3rd Generation Partnership Project (3GPP), which is rapidly spreading recently, the HSDPA system has been proposed, and studies and developments thereof are being promoted (for example, refer to a non-patented document 1).
In the HSDPA system, the HS-SCCH and a High Speed Physical Downlink Shared Channel (HS-PDSCH) are stipulated as physical channels for downlink, through which data is transmitted from a base station to a terminal, and the HS-SCCH and the HS-PDSCH are utilized as a pair. In the HSDPA system, the HS-PDSCH transmits packet data for the HSDPA, while the HS-SCCH transmits decode information used for decoding the HS-PDSCH packet data transmitted in pair.
Meanwhile, as a physical channel for uplink through which data is transmitted from the terminal to the base station, a High Speed Dedicated Control Channel (HS-DPCCH) is stipulated. A structure of a sub-frame of the HS-DPCCH is shown in FIG. 6.
As shown in FIG. 6, the sub-frame of the HS-DPCCH includes Hybrid Automatic Repeat reQuest-ACK (HARQ-ACK) information that notifies the decoding status of the HS-PDSCH by Acknowledgement (ACK)/Negative Acknowledgement (NACK)/Discontinuous Transmission (DTX), and a Channel Quality Indicator (CQI) that indicates reception quality of the data.
Regarding the HARQ-ACK, in the case where the terminal detects the HS-SCCH addressed thereto and a CRC result of the HS-PDSCH is good, the ACK is inserted; in the case where the terminal detects the HS-SCCH addressed thereto and the CRC result of the HS-PDSCH is no good, the NACK is inserted; and in the case where the terminal has not detected the HS-SCCH addressed thereto, neither is inserted, in other words the DTX is selected.
The CQI represents numerical values of 0 to 30 indicating the reception quality at a Common Pilot Channel (CPICH) of the data transmitted from the base station to the terminal, which are mapped on the sub-frame of the HS-DPCCH. According to the 3GPP, lower CQI values indicate that the reception quality is worse, and higher CQI values indicate that the reception quality is better.
In the operation of the HSDPA system, the base station transmits, upon receipt of the HS-DPCCH transmitted from the terminal, and according to the HARQ-ACK information, a new packet in the case of ACK, or retransmits in the case of NACK, and the retransmission is also executed in the case of DTX and where the HSDPA communication has been performed to the corresponding terminal. Then the base station decides the number of codes, a Transport Block Size (TBS), and a modulation method, so as to transmit the HS-PDSCH appropriate for the reception quality based on the CQI values mapped on the HS-DPCCH transmitted from the terminal. Thus, the HS-SCCH and the HS-PDSCH are transmitted as a pair.    [Non-patented document 1] “High Speed Downlink Packet Access (HSDPA)” [3GPP TS 25.308 V7.0.0 (2006-03)]